The LENA team study the atomic nuclei under extreme conditions of deformation, in spin, isospin or mass, with the objectives of testing with precision the validity of the nuclear models through the comparison of the calculations with the structure data obtained under these extreme constraints on the nuclear systems.
In particular, the shape of the nucleus may vary strongly by adding or removing a nucleon, or by increasing the nucleus spin. The nuclei may also present various deformation shapes at very close excitation energies, this is called shape coexistence. The shell structure also plays an important role in the development of deformation in nuclei with open shells. The understanding the evolution of nuclear shapes and the occurrence of shape coexistence is an important objective.
The experimental tools are large gamma-ray detectors, such as EXOGAM and the recently developed AGATA array, often coupled to particle spectrometers. Coulomb excitation and lifetime measurements in the picosecond range using the so-called "plunger" method are primary means in order to determine observables linked to the nuclear shape, such as static and transitional quadrupole moments.
A part of the experiments led by the LENA team was performed at the Legnaro facility. The group has studied the neutron-rich Zn nuclei close to the new third "island of inversion" around neutron number N=40 and started a detailed investigation of the sudden onset of strong deformation at N=60 in nuclei around Z=40. The first experiment using the AGATA demonstrator at the Legnaro-INFN National laboratory (LNL, Italy) showed an almost unique and yet to understand low collectivity of 4+ states as compared to the usual rotational or vibrational behaviour. The IRFU teams have been strongly involved in the development of the AGATA project (Advanced GAmma-ray Tracking Array), this spectrometer is now in its exploitation phase.
The second experiment was performed at GANIL, where the large-acceptance VAMOS spectrometer was used to identify fragments from in-flight fission and the EXOGAM spectrometer to measure lifetimes of excited states in several tenths of isotopes.
The group has also used EXOGAM for an experiment performed at the ILL (Institut Laue Langevin) reactor at Grenoble. The goals were to collect a complementary data set of fission fragments using the EXOGAM spectrometer in combination with LaBr3 detectors, to determine longer lifetimes than with the plunger method. First interesting features in the evolution of collective properties have shown up, which clearly demand more refined nuclear model calculations including configuration mixing of all relevant degrees of deformation (i.e. tri-axiality). Complementary experiments used the Coulomb excitation technics, e.g. on 100Zr at the Argonne National Laboratory (USA).
In the long-term range plan, this technics will also be employed to measure the spectroscopy of neutron-rich nuclei at the HIE-Isolde accelerator of CERN or at SPIRAL2.
Page of the AGATA collaboration: http://www.agata.org/
PAGE AGATA at GANIL: http://pro.ganil-spiral2.eu/spiral2/instrumentation/agata